It is known to arrange an afterburning device as close as possible to the engine to provide the gases with no opportunity to cool on the way to the afterburner, especially in a cold start. The primary purpose of this proposal is to have the afterburning of the exhaust established promptly after the cold start. This arrangement is especially important after a cold start, when the engine is still comparatively cool, because the exhaust gases contain a large proportion of noxious constitutents in this particular operating mode. After operating temperature has been reached, when the noxious constitutents in the exhaust diminish, since the thermal load capacity of the afterburning device is generally low, the exhaust flow is passed for the most part through the by-pass line by means of a control actuated in response to heating and usually arranged in the by-pass line.
To a known afterburning system, (see German DT OS No. 2,360,581) a throttling flap alternately opens and closes the by-pass line and the exhaust line to the afterburner, to avoid continuing to expose the afterburner to the hot exhaust even when the flow is being carried by the by-pass line and thus shortening its life. However, not only does such a flap in each case set transverse to the exhaust flow, adversely affect the flow of exhaust gas, but also a considerable distance must be kept between the engine and the afterburner for the installation of the flap, so that after a cold start the heating time of the afterburner is undesirably prolonged and the afterburning of the exhaust occurs correspondingly late. Besides, the exhaust ducts leading to the afterburner, as well as the by-pass line, are seriously affected by engine vibration, which tends to cause early failure thereof.